From the aspect of protecting the environment not only in local areas but also in the entire globe, exhaust gas-cleaning catalytic converters and particulates-capturing filters with ceramic honeycombs are used to remove harmful substances such as carbon particulates, etc. from exhaust gases emitted from diesel engines. As shown in FIG. 1, a honeycomb structure 1 integrally comprises a peripheral wall 3 and cell walls 4 defining a large number of cells 6. The honeycomb structure 1 is strongly gripped by a grip member disposed between an inner surface of a metal container (not shown) and the peripheral wall 3 of the honeycomb structure 1.
The honeycomb structure 1 is conventionally produced according to the following steps. Cordierite-producing ingredient powder, a shaping aid, a pore-forming agent and water are kneaded to produce a soft ceramic material, which is shaped by extrusion to provide a green body having a honeycomb structure, in which the peripheral wall 3 and the cell walls 4 are integral with each other. After drying and removing the shaping aid such as a binder, etc. by heating, this green body is fired to provide a honeycomb structure 1 integrally composed of a finely porous peripheral wall 3 and finely porous cell walls 4, which have the predetermined shape and strength.
However, in the case of producing large ceramic honeycomb structures of 150 mm or more in outer diameter and 150 mm or more in height for diesel engines, and thin honeycomb structures with cell walls 4 as thin as 0.2 mm or less, for instance, the green bodies are too heavy or insufficient in strength to support themselves, causing the problem that the cell walls 4 near the peripheral wall 3 are collapsed or deformed.
As a method for solving this problem, Japanese Patent 2,604,876 discloses a method of kneading cordierite ingredients with a shaping aid and/or a pore-forming agent, shaping the resultant soft ceramic material by extrusion, drying and firing the resultant green body to provide a fired body having a honeycomb structure, removing a peripheral wall 3 and nearby cell walls from this fired body having a honeycomb structure by grinding, coating the exposed cell walls with a coating material, drying and curing it to form a peripheral wall layer. In this method, because the peripheral wall 3 and the nearby cell walls 4 are removed from the fired body having a honeycomb structure by grinding, the peripheral wall 3 and nearby deformed cells can be removed. When a fired body having a honeycomb structure has low roundness, the peripheral wall layer is formed after increasing the roundness by grinding. Accordingly, it is possible to improve the dimensional accuracy of the honeycomb structure. If coating materials containing ceramic fibers and inorganic binders were used, it would be possible to provide the peripheral wall layer with high strength.
As a ceramic honeycomb structure having a peripheral wall layer with improved peeling resistance, Japanese Patent 2,613,729 discloses a ceramic honeycomb structure having a peripheral wall layer composed of cordierite particles and/or ceramic fibers, and an amorphous oxide matrix comprising colloidal silica or colloidal alumina.
One example of the structure of a ceramic honeycomb 1 is shown in FIGS. 2 and 3. A ceramic honeycomb body 10 comprises a large number of cells 6 partitioned by cell walls 4, and axial grooves 14 formed on the outermost circumference of the ceramic honeycomb body 10, the grooves 14 being coated with a peripheral wall layer 12, which does not peel off from the honeycomb body 10 during use, while suppressing decrease in the thermal shock strength of the honeycomb structure 1.
It has been found, however, that when the ceramic honeycomb structures described in Japanese Patents 2,604,876 and 2,613,729 are used for catalytic converters for cleaning an exhaust gas and filters for capturing particulates, there arise the following problems.
When the ceramic honeycomb structure is used as a catalyst carrier and a particulate-capturing filter, the ceramic honeycomb structure is firmly gripped by a grip member in a metal container. Because a high-temperature exhaust gas flows through a larger number of flow paths in the ceramic honeycomb structure, rapid temperature elevation locally occurs particularly at the time of starting operation, causing temperature difference between the center portion and the peripheral wall layer of the ceramic honeycomb body, and thus generating thermal stress in the ceramic honeycomb structure, which sometimes leads to cracking in the peripheral wall layer. Also, when used as a particulate-capturing filter, local temperature elevation occurs particularly near the center portion of the honeycomb structure in which a large amount of particulates are accumulated, by heat generated by a regeneration treatment for burning particulates accumulated in the filter, resulting in cracking in the peripheral wall layer by thermal stress.
If cracking occurs in the peripheral wall layer and propagates to the cell walls, the cell walls are detached, resulting in decrease in the performance of cleaning an exhaust gas, and also decrease in a particulates-capturing rate because of the communication of flow paths between the inlet and outlet.
When a large honeycomb structure having an outer diameter of 150 mm or more and a length of 150 mm or more or a thin honeycomb structure with cell walls having a thickness of 0.15 mm or less is produced, an extrusion-shaped green body is likely to have defects such as breakage or deformation in cells near the peripheral wall. As a result, it has a remaining stress provided by shaping and drying. If firing is carried out with such defects, cracking is likely to propagate to free the remaining stress from defective portions, and spread in the entire fired body. Some cracks are not completely eliminated by removing the peripheral wall and the nearby cell walls from the fired body, resulting in decrease in a production yield.
Because the fired ceramic honeycomb is hard and brittle, as shown in FIG. 4, chipping 4a easily occurs in the outermost cell walls 4 constituting the grooves 14, and part of the cell walls 4 are easily cracked, resulting in defective grooves 14a. Because the grooves 14 on the circumference have a small contact area with the cell walls 4, the honeycomb structure 1 is likely to suffer from decrease in isostatic strength and the peeling of the peripheral wall layer 12. When such a honeycomb structure is used for catalytic converters and particulate-capturing filters, the peripheral wall layer peels off from the honeycomb structure by the vibration of an engine and vibration by contact with the road, so that a proper holding force cannot be maintained in the container, failing to keep the honeycomb structure from moving in the container, and thus resulting in the breakage of the honeycomb structure.
Japanese Patent 2,604,876 uses a grinder rotating at a high peripheral speed of 750 to 2100 m/minute to remove the peripheral wall from the fired ceramic honeycomb, to conduct grinding at a speed of 0.7 to 0.9 mm/minute. However, because the grinding of the cell walls of the honeycomb structure is an intermittent working, in which a tool impinges the cell walls intermittently, the extent of working such as feed and depth of cutting, etc. must be kept small, resulting in the long working time. In addition, because the fired body is hard and brittle, an expensive grinder such as a diamond grinder should be used.
In the above prior art, because a coating material comprising cordierite particles and an inorganic binder is applied to the outer surface of the honeycomb body made of cordierite to form the peripheral wall layer, the peripheral wall layer has a larger thermal expansion coefficient than that of the honeycomb body, resulting in a tensile stress remaining in the peripheral wall layer and a compression stress remaining in the cell walls after drying and firing. The reason why the peripheral wall layer has a larger thermal expansion coefficient than that of the honeycomb body is that because kaolin particles (hexagonal planar crystal) in the material passing through the narrow slits of an extrusion die slit are oriented in plane (in the walls), and because a hexagonal columnar cordierite crystal is orientated perpendicularly to the orientation direction of kaolin by firing, the cell walls have a small thermal expansion coefficient, while the peripheral wall layer is composed of the randomly orientated cordierite particles and the inorganic binder having a large thermal expansion coefficient.
When the honeycomb structure is gripped for the purpose of being introduced into the container, too, tensile stress may occur in the peripheral wall layer. When rapid temperature elevation occurs in the center portion of the honeycomb structure while the tensile stress exerting onto the peripheral wall layer, tensile stress in the peripheral wall layer increases due to the temperature difference between the cell walls and the peripheral wall layer, making it likely that cracking occurs in the peripheral wall layer.